Persist fast mode across new threads

[nathansobo]

When toggling fast mode, the setting is now written to settings.json under agent.default_model.speed, so new threads start with the same speed. This follows the same pattern as enable_thinking and effort.

The speed field uses the existing Speed enum ("fast" / "standard") rather than a boolean, to leave room for future speed tiers.

Example settings:

{
  "agent": {
    "default_model": {
      "provider": "zed.dev",
      "model": "claude-sonnet-4",
      "speed": "fast"
    }
  }
}

cc @benbrandt

Release Notes:

• N/A